not find any antigenic differences with these species. In mammalians we know there is no
antigenic crossreactivity between different species. Now, to what extent the species
should be called subspecies, I do not know, but several tilapias cannot be termed different
species per se if we refer to immunological criteria. In this respect, we found some differ-
ences between T. rillii and Sarotherodon species, but no differences between s. galilaeus,
S. aureus, and S. niloticus. However, this matter needs to be studied in more detail.
LOWE-McCONNELL : So the particular question is, do you think that the latter are truly
separate species as you have been unable to find any immunological differences?ROBERTS: I think we will have to distinguish between antigenic characteristics (which
means tissue matching) and serum and muscle enzymes, because there are certainly
differences between all of the Sarotherodon species in terms of the numbers of enzyme
loci.HEPHER : But you can also find such differences between sexes.ROBERTS: Yes, this has been reported, but not to anything like the same degree as the
differences between species.HEPHER : But there are differences.
ROBERTS: We did not find differences between sexes. I think the important point is
that, while someone like Dr. Trewavas has devoted so much time to distinguishing mor-
phologically between these different species, Dr. Avtalion, from his antigenic and immu-
nological studies, finds that there is no difference. I think we have to be very careful with
our definition of a species and recognize a problem here.LOWE-McCONNELL: This is a particular subject for people who are interested in
speciation.PULLIN: Dr. Avtalion, you said that you found a large difference between T. zillii
and Sarotherodon species which tends to support the generic split.AVTALION: Yes, there is quite a significant difference.TREWAVAS: It is interesting that you bracket both kinds of Sarotherodon, that is,
S. (Sarotherodon) galilaeus together with S. (Oreochromis) niloticus and aureus.HEPHER: We have noticed that it is very easy to hybridize S. niloticus and S. aureus,
but it is very difficult to cross S. galilaeus with S. aureus or S. niloticus. Maybe this has
some significance.LOWE-McCONNELL: I think the important thing here is that there are "tilapias," there
are "sarotherodons" and there is a group in west Africa that are more difficult to define.
We do not know much about these.TREWAVAS: Yes, I think that S. galilaeus is the only species that fish culturists have
had anything to do with from that group.